The control of eye movements represents one of the most basic and fundamental motor control systems of the human brain, combining information from both low-level motion transducers in the inner ear (vestibular system) and high level-visual information. The measurement of eye movements has become an important tool in probing some higher-level cognitive functions, including reading, learning, prediction, memory, and direction of attention. Understanding and developing neural models of eye movement control systems is helpful not only to the clinical neurologist in aiding with the diagnosis and localization of functional deficits, but also to the wider neurological research community in contributing to the general understanding of how the human brain directs visual attention and controls motor systems. As more is learned about how the various parts of the brain operate and interact, precise measurement of eye movements has been called on more and more to provide both direct and indirect assessment of vestibular, oculomotor, visual, and neurologic function. To this end, increasingly sophisticated experiments and more inexpensive and versatile equipment have been developed.
There are a number of widely used methods for measuring eye position, each with its own benefits and drawbacks. One of the most popular methods used for the eye measurement is the scleral search coil system, developed by David A. Robinson (Robinson DA. A method of measuring eye movement using a scleral search coil in a magnetic field. IEEE Trans Biomed Electron 1963; BME-10: 137-145). Among its benefits are high resolution and accuracy, low noise, fast response (wide bandwidth), wide range, cost effectiveness, ease of use, and ability to measure torsional eye movements (rotations about the line of sight).
In existing systems embodying such methods, a contact lens containing one or two coils of wire is placed on the subject's eye or the coil(s) is surgically implanted for animal studies. The coils on the contact lens are connected to electronic amplifiers through fine wires that extend from the lens. The subject sits inside a set of large coils that generate relatively homogeneous AC magnetic fields. These magnetic fields induce current in the coil on the eye, which are detected and amplified and typically sampled by a computer. Since the magnitude of the signal induced into the eye coil is proportional to the sine of the angle between the plane of the eye coil and that of each magnetic field, one can deduce the orientation of the eye from the magnitudes of the received signals.
There are many variations on the search coil system. Such systems can use anywhere from one to three generated magnetic fields, and one or two coils on the eye. Using a single coil on the eye with its plane perpendicular to the line of sight, it is possible to deduce the direction of gaze (i.e., the horizontal and vertical position of the eye). When a second coil perpendicular to the first coil is provided, it is possible to deduce the amount of torsional rotation about the line of sight. In sum, the original search coil system has been in use since the early 1960s with only minor modification and it continues to be widely used when detailed and precise eye movement information is required. Although the scleral search coil system is presently considered the standard, it is not without its limitations as described below.
As indicated above, a wire extends from the lens/coil assembly to the instrumentation. This wire is delicate and is a major source of coil defects. It is not uncommon, especially in a subject who blinks often while making eye movements, for the wire to break at the interface with the lens/coil.
Also, this wire is a source of discomfort for the subject as it rubs the edges and inner surfaces of the eyelids and may become entangled in the eyelashes. In addition, this wire can contribute to motion artifacts, since the eyelids can push the wire and, therefore, move the coil and lens on the eye (Bergamin O, Roberts D C, Ramat S, Straumann D, Zee D Z. Influence of orientation of exiting wire of search coil annuli on torsion following saccades. AR VO Abstracts 2003).
The wired scleral contact lenses used in humans are fragile and expensive and typically last for only a few recording sessions. Experiments typically call for a coil on each eye, so a continuous and costly supply of these lenses must be maintained. Further, wire breakage during experiments causes delays, loss of data, and sometimes, due to time constraints, early termination of the experiment.
Surgically implanted coils used in animal studies are extremely expensive due to their risky and labor intensive nature and are also susceptible to breakage. The wire must extend from the eye, under the skin, to a connector on the animal's head. This is an added source of infection and malfunction. Rubbing of the wire inside the animal's head can be a source of discomfort and infection. The mechanical load of the wire can also cause eye movement problems and strabismus.
The standard system using scleral search coils also employs a large cubical frame to hold the field-generating coils, ranging from one to two meters for humans, to several tens of centimeters for smaller animals. An example of a such a cubical frame for human use is shown in FIG. 1. For accurate recordings the eyes must remain near the center of the cube, and thus, the head must be fixed with respect to the surrounding field coils. Also, less head and body movement is allowed as the size of the cube gets smaller. This is such a great restriction that as a general rule, no practical attempt is made to make head-free measurements while using search coils.
The large field coil structure also imposes limits on the location of coil experiments. Typically, such coil systems are not portable. The field coils are usually permanently mounted to a chair or other object in which the subject is confined. Thus, the person or subject must be in the chair or other object and remain in that chair during the entirety of the procedure. As indicated above, once in the chair the movement of the subject also is usually severally limited. All of this severely restricts the use of search coil systems in novel locations, situations, and orientations.
Another potential problem with existing search coil systems is slippage of the contact lens in human subjects, especially torsionally. Torsional slippage is usually reported during blinks (Teiwes W, Merfeld D M, Young L R, Clarke A H. Comparison of the scleral search coil and video-oculography techniques for three-dimensional eye movement measurement. In: Three-dimensional kinematics of eye, head, and limb movements, M Fetter et al. (eds.), Harwood Academic Publishers, Amsterdam, 1997), indicating that lens slippage may be due to the eyelids pressing on the protruding wire.
Another major confounding effect is that scleral coils seem to change the neural command signal to the extraocular muscles (Frens M A, van der Geest J N. Scleral search coils influence saccade dynamics. J Neurophysiol 2002; 88:692-698). For example, saccades are longer and slower when coils are worn (though the effects are each less than 10%). Furthermore, this occurs in both eyes even with a coil in only one eye, so it is not a purely mechanical phenomenon. Without being bound to any particular theory, it is believed that some of this effect may be due to irritation of the eyes from the coils, and in particular from the connecting wires leading from the scleral coils as this is almost always the part of the coils that cause the most discomfort and “awareness” of something in the eye. An ill-positioned connecting wire tends to slide across the lower lid or the lashes, which can be quite annoying and could easily lead to a desire to reduce eye velocity and the duration of eye movements.)
The disadvantage of the wire running from the scleral eye coil to the instrumentation has been recognized by several groups. One approach taken for a search coil that does not use an interconnecting wire that has been proposed (Bour L J, van Gisbergen J A. The double magnetic induction method for measuring eye movement-results in monkey and man. IEEE Trans Biomed Eng 1984; 31:419-27; Bos J E, Reulen J P H, Boersma H J, Ditters B J. Theory of double magnetic induction (DMI) for measuring eye movements: correction for nonlinearity and simple calibration in two dimensions. IEEE Trans Biomed Eng 1988; 35:733-9; Reulen J P, Bakker L. The measurement of eye movement using double magnetic induction. IEEE Trans Biomed Eng 1982; 29:1404), embodies a methodology that relies on the double magnetic induction (DMI) property of conductors in a magnetic field. It uses a simple short-circuited loop of wire (or a metal ring) on the eye. An external magnetic field (the primary field) induces a current in the loop on the eye. The loop on the eye, in turn, generates its own magnetic field (the secondary field), which is detected by a second detector near the eye.
The secondary magnetic field emanating from the shorted eye coil is of the same frequency as, but in phase quadrature to, the primary magnetic field (i.e., they are 90° out of phase), making it possible to sense and detect the secondary field in the presence of the primary field by using a phase-sensitive detector. This elegant solution is useful only for horizontal and vertical eye movements, and cannot be easily extended to include torsional measurements. Small movements of the eye relative to the secondary detector coil cause large errors in the measurements. The DMI system still constrains the subject's position within a large field coil frame. Moreover, these systems have seen only very limited use in a few labs.
Other commercial devices currently in use for the measurement of eye movements include electrooculography (BOG), the infrared (IR) eye tracker, and the video eye tracker (also called VOG for videooculography). BOG, which uses electrodes placed near the eyes, is relatively easy to use, but suffers from lack of sensitivity due to muscle noise, drift due to changes in the skin-electrode interface, and changes in gain and bias due to changes in corneo-retinal potential with variations in ambient lighting. EOG is unsuitable when high resolution and accuracy are required, it cannot measure torsion, and it provides poor vertical measurements. The IR (infra-red light reflection) monitor is easy to use and is insensitive to muscle artifact. However, most IR tracker systems cannot accurately measure eye movements greater than approximately ±15 to ±20 degrees horizontally, and less vertically. These systems also can be difficult to calibrate, and do not measure torsion.
VOG based systems use a small camera and digital image processing hardware and software to compute the eye's orientation. Most systems limit the sample rate to 50 or 60 Hz, though some newer systems are beginning to use digital cameras with higher sampling rates. No video based system today approaches the search coil's ability to measure horizontal, vertical, and torsional eye movements at 1000 Hz and above.
In addition to these limitations, an inherent limitation of VOG is that a camera needs to acquire an image of the eye, and in some situations this maybe difficult or impossible. Droopy eyelids or narrow eye openings cause problems for VOG, as do blinking, squinting, and, obviously, closed eyes. Wearing glasses is not permitted with many types of VOG systems, and patching or occluding an eye may not be permitted. A camera, or some form of optics or right angle prisms, must always be in the field of view, which can be unacceptable in many circumstances. The use of a stereoscopic video apparatus to present visual stimuli (VR: virtual reality) would present a problem, unless the apparatus was specifically designed to accommodate VOG.
There are also several very common experimental setups in which a video system is inconvenient or unacceptable. It is not uncommon to evaluate patients with fourth nerve palsies, which cause the eyes to be misaligned. Part of the testing procedure involves having the patient look at various targets while the right and left eyes are alternately covered, and the positions of both eyes are continuously monitored. This may be problematic because, depending on the configuration of the particular VOG system, covering of the eye may preclude the recording of its position. Another set of experiments requires the subject to wear thin prisms which alter the angle of the image reaching the eye. Video systems would at the least complicate the determination of eye position, or at worst not be usable at all.
Illumination of the eye is required for the camera to capture an image, however, many evaluations require absolute darkness so that the subject has no fixation targets. While infrared illumination results in decreased visibility of the illumination, it may not eliminate it. IR LEDs are sometimes visible as a dim red glow to a subject that is dark adapted, and this can compromise the conditions of the evaluation.
In addition, it can be very difficult to get an image of the eye that is good enough to perform torsional eye movement analysis. Proper torsional analysis requires not just a sharp image of the pupil (which is relatively easy to acquire), but a sharp image of other features of the eye (e.g., the iris) that will allow automatic tracking of torsional movements. The quality of the image can depend on the color of the iris, and the shape of the eyelids affects how much of the iris is visible, so the quality of torsional movements can vary widely between subjects. Proper illumination also is important in obtaining a good iris image. Higher-wavelength IR illumination is less visible to the subject, but it also results in a more washed-out (lower contrast) image of the iris, leading to poorer torsional tracking.
Another shortcoming with VOG is the need to mount the imaging camera to the subject's head, possibly interfering with head motion. Furthermore, if the camera is not very firmly attached, jitter in the resulting image will degrade the quality of the measurement. This is a significant problem that severely restricts the use of video in situations where free and rapid head and body motions occur. The more effective methods to stabilize the device on the head are also quite uncomfortable, requiring a biteboard or clamping to the head.
It thus would be desirable to provide a new scleral search coil including system and methods related thereto which would overcome the shortcomings of the prior art device that uses wires to interconnect the coil to the instrumentation. It would be particularly desirable to provide such a coil, systems and method that would allow the coils and systems to be portable and thus capable of being used in other than a confined subject situation as when using known scleral search coils and related systems. Such search coils preferably would be simple in construction and less costly than prior art scleral search coils.